This invention relates generally to a gate valve having a fail safe actuator. More particularly, it relates to a gate valve of this type which is especially useful in a subsea environment at considerable depth below the water surface.
In a typical fail safe valve, a spring acts between the valve stem and an actuator mounted on the body of the valve in which the gate reciprocates to urge the gate to one position, which may be closed, and control fluid is applied to a piston on the stem reciprocable within a pressure chamber in the actuator to overcome the force of the spring and thereby maintain the valve in its other position, usually open, so long as a controlled condition is maintained. If, however, the condition "fails", the control pressure is exhausted to permit the spring to move the gate to its closed position. Typically, the condition being controlled may be pressure within the line in which the valve is installed, or another pressure condition at a location remote from the valve.
In the event the body pressure is significantly greater than ambient sea pressure, it provides a net outward force on the stem to move the gate to its failed position upon exhaustion of control pressure. In this situation, the spring may not even be needed as long as the net force due to body pressure acting on the stem is sufficient to overcome the above described forces. If, however, body pressure is less than, or even approximately the same as, ambient sea pressure, the spring is required.
When the valve is disposed in subsea environments, combined forces due to the ambient sea pressure at the subsurface level and the static control fluid head pressure urges the stem piston inwardly, and thus the gate to its controlled position with substantial force. Hence, in order for the valve to move its failed position, upon the exhaust of control pressure, the force due to the spring and that due to body pressure acting on the stem must be great enough to overcome the force on the stem due to ambient sea pressure, the force on the piston due to the control fluid's hydrostatic head pressure, plus any friction forces.
Thus, when a valve of this construction is disposed at great depth, and body pressure drops significantly below the ambient sea pressure, the spring must be large enough to close the valve as the force due to body pressure acting on the stem will not be sufficient to move the gate to its failed position. The extreme condition could occur, for example, when the valve is used in a "block and bleed" environment where the body pressure goes to zero. In this situation, a small diameter stem is preferred in order to decrease the net force on the stem, thereby reducing the size of the actuator and/or increasing the depth rating of the valve. Conversely, when the body pressure is significantly above the ambient sea pressure, a large diameter stem is desired.
The object of this invention is to provide a valve of this type which is of such construction as to move to failed position under either condition, i.e., regardless of which pressure is significantly greater, while also minimizing the size of the actuator.
This and other objects are accomplished, in accordance with the illustrated embodiment of the invention, by a fail safe valve which includes, as in conventional valves of this type, a valve body having a flowway therethrough and a cavity intersecting the flowway, a gate reciprocable within the cavity between positions to open and close the flowway, and an actuator for moving the gate between such positions. As is also the case in valves of this type, the actuator comprises a bonnet mounted on the valve body and a stem connected to the gate and extending sealably out one end of the bonnet and having a piston sealably slidable within the bonnet to form a pressure chamber on one side thereof, whereby control fluid may be supplied thereto so as to move the gate to and hold it in one position, usually open, as long as the controlled position is maintained. Spring means acting between the actuator and stem as well as ambient pressure acting on the other side of the piston urge the gate toward its other closed or failed position, so that, upon the exhaust of control pressure in response to loss of that condition, the gate is moved to its one or open position.
In accordance with one illustrated embodiment of the invention, a floating ring is sealably reciprocable within an annular space between the bonnet and stem between one limited position engaged with a stop on the bonnet, when the body pressure is below a predetermined level, and an opposite limited position engaged with a stop on the stem, when body pressure is above said level. Thus, where the pressure in the body significantly exceeds the ambient sea pressure, and the ring is caused to bottom out against the shoulder on the stem due to the upward force produced by the pressure differential across it, the stem and ring behave like a large diameter stem. On the other hand, when the body pressure in the gate valve is significantly less than the ambient sea pressure, the stem and ring behave like a small diameter stem in that the ring will be pushed down against the stop on the bonnet due to the pressure differential in the downward direction, whereby the effective cross sectional area of the stem is only from the small stem diameter that contacts the inside diameter of the floating ring. Thus the opposing force that the actuator spring must overcome in order to close the gate valve is reduced since the force produced by the sea pressure acting on the floating ring is carried by the bonnet.
In accordance with a second illustrated embodiment of the invention, a balance stem extends sealably out a lower housing or the opposite end of the main housing of the actuator for reciprocation between an inner position engaged with gate opposite the operating stem and a limited outer position engaged with the lower housing, together with spring means acting between the balance stem and lower housing to urge the balance stem toward engagement with the end of the gate. This also has the effect of a large stem when the gate valve body pressure is substantially higher then the ambient sea pressure. On the other hand, when the body pressure is not substantially higher than the ambient sea pressure, this design can give the net effect of a small diameter stem, a zero diameter stem or even a negative diameter stem as the spring means on the balance stem is designed to have a substantial effect on the balance stem.
This design differs from prior designs wherein a spring means acting on a detached balance stem has been used only to overcome frictional drag forces from seals in the bore, and instead provides much higher force on the balance stem and thus allows operation at greater depths. Prior designs of balance stems also rely on downstream bore pressure acting directly on the spring means of the balance stem. The present invention eliminates the need of such downstream pressure assistance by providing spring means of such design that its effect on the detached stem substantially modifies the effects of the ambient sea pressure.
When closing the gate valve, where the body pressure in the gate valve is significantly greater than the ambient sea pressure, the gate valve functions like a regular gate valve that has no balance stem in that the balance stem will simply move out until it engages the stop on the valve body. However, when the pressure inside the valve acting on the balance stem is less than the combined forces of the ambient sea pressure acting on the outer end of the balance stem and the spring force, the detached balance stem is raised to bear against the bottom of the gate with a force produced by the balance stem spring and the differential pressure acting on the balance stem.
In the case where the spring energized detached balance stem is used with an existing subsea actuator, the force produced by the actuator, as the valve is closed, would increase significantly to allow this actuator to be used in deeper water. Present uses of a spring energized detached balance stem do not allow for the use in deeper water due to the spring being designed merely to overcome frictional drag forces.
As an alternative option to increasing the actuator's closing pressure, the minimum closing pressure can be kept about the same and the actuator's main spring size can be decreased. Yet another option is to size the balance stem spring and the main actuator spring to act independently to close the gate valve, regardless of the bore pressure, and without help from the other spring. This is a significant feature because, if the actuator spring breaks in a standard subsea actuator, the valve will not fully close when there is low body pressure in the gate valve.
In accordance with a third illustrated embodiment of the invention, both the first and second embodiments are combined. Thus, a floating ring is sealably reciprocable within an annular space between the bonnet and stem between one limited position engaged with a stop on the bonnet and an opposite limited position engaged with a stop on the stem, a balance stem extends sealably out of the lower bonnet on the other end of the main housing for reciprocation between an inner position engaged with the gate and a limited outer position engaged with the lower bonnet, and spring means acts between the balance stem and lower bonnet to urge it toward engagement with the gate. This then has the advantages of both the first and second embodiments as well as the ability to produce a greater closing pressure when the body pressure its lower.
As illustrated, the valve of the present invention is of the fail closed type wherein the gate is held in open position as long as control pressure is maintained, and moved to closed position when the controlled condition fails.